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Introduction to Vacuum Tubes

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Introduction to Vacuum Tubes Introduction to vacuum tubes 1. Operating principles and manufacturing technologies 2. The collection 3. Go to the ‘Tube List’ 1. Operating principles and manufacturing technologies The operation of electron tubes, also referred to as vacuum tubes, depends upon the current associated to a stream of electrons, negative charged elementary particles, flowing from the surface of an electrode, referred to as cathode, to another electrode, called anode. The emission of electrons takes place from the surface of the cathode whenever electrons, under the action of different energy sources, are accelerated above the so called ‘work function’, characteristics for each metal and usually expressed in volts. The type of emission depends upon the source of energy used to accelerate electrons: - Thermoionic, or primary emission, is the most used in electron tubes and takes place when cathode is heated near to the incandescence. Usually there is a filamentary resistor heated to a temperature going from dull-red to white by a current flowing in it. Alternatively the emitting surface can be heated by other means, as electron or ion bombardment. - Secondary emission, when high-energy particles strike the cathode surface. - Phothoelectronic emission, when surface electrons are excited by visible, IR or UV radiations. - Field emission, when electrons are accelerated at room temperature by intense electric fields. The cathode base materials and its surface coating if any, as well as the source of heat and the heat transfer to the cathode itself, are selected for optimum operation in each tube type. In free air emitted electrons are readily blocked by impacts with gas molecules. Anyway they easy travel to a positive anode electrode in high vacuum, resulting in a current flow in the cathode-to-anode space. Vacuum tubes then must have a sort of hermetic envelope, carefully evacuated and sealed after the electrodes have been assembled. In some cases a little quantity of gas may be intentionally pumped into the envelope. In these tubes some electrons impact gas molecules, so originating positive charged ions that flow to the cathode and contribute to increase the conduction. In absence of superimposed electric fields, electrons emitted from the cathode surface originate a dynamic electron cloud all around. The cloud, known as space charge, repels other electrons emitted by the cathode and at the same time gains part of their energy. Electrons emitted from the cathode are attracted by a positive charged metal plate placed inside the same envelope. In their travel to the plate electrons generate a current flow. A very weak current flow to the plate can be observed even in absence of any electric field. This current is generated by electrons escaping the space charge cloud, due to the energy impressed them by the cathode thermal emission. Anyway we will only consider the currents flowing in the vacuum tube when the plate is sufficiently positive with respect to cathode to overcome the repelling effect on electrons by the space charge. Fig. 1 - A pictorial representation of electrons moving from the cathode cylinder (-) to the plate (+) under the effect of the electric field generated by the plate battery. The double spiral heater inside the cathode supplies the thermal energy needed to accelerate electrons and let some of them to leave the cathode surface. On the right a typical plot of the plate current versus plate voltage. The non-linearity and the offset at very low voltages are due to the presence of the negative space charge, the cloud of free electrons surrounding the cathode, which sums algebraically to the field generated by the plate battery. Quite soon a saturation point S is reached where the current does not increase further, since all emitted electrons are attracted by the anode. The flow of electrons in their travel from the cathode to the anode, under the influence of a steady electric field applied between these electrodes, can be controlled by superimposed electric or magnetic fields. The grid is the electrode capable of controlling electron flow by electric influence. Usually it is a spiral of very tiny wire placed around the cathode. Historical notes Extensive experiments on conduction across electrodes as function of electric fields, gas pressure and temperature were performed by Edmond Becquerel as early as 1853. Since 1880 Edison started to investigate the reason why the bulb of his lamps darkened during operation. To detect the particles that were supposed to be responsible for glass darkening, Edison added a small metal strip in the bulb of some samples. When the plate was connected to the positive terminal of the filament, he noted a current flow from the filament to the metal plate proportional to the degree of incandescence, or the temperature, of the filament. He concluded that carbon particles moved toward the glass under the action of negative charges, later known as electrons. The so called ‘Edison effect’, that was the forerunner of thermoionic emission in vacuum tubes, was then thoroughly investigated even in other countries, mainly to increase the efficiency of light bulbs. In November 1883 Edison applied for a patent on an ‘Electrical Indicator’, a lamp capable of giving remote indication of its operation, just monitoring the current flowing to a metal strip added in the bulb. John Ambrose Fleming ran further experiments, even using alternate currents, on Edison effect when working at the Edison Electric Light Company of London. In 1899 Fleming became technical advisor for the Marconi Wireless Telegraph Company. Looking for a sensitive detector of radio signals he successfully tried the lamps used for his experiments on the Edison effect. He called the device ‘oscillation valve’ and patents in Great Britain, U.S. and Germany were filed between 1904 and 1905. This can be considered the first use of a thermoionic vacuum tube and the beginning of the electronic science. The next step was to investigate devices in which a signal could control the electron flow, capable of operation either as amplifiers or as oscillators. Small quantities of gas inside the envelope, often due to poor vacuum pumps available, were also proposed to increase the sensitivity of some devices when used as detectors, biased close to the ionization threshold. Several ways were tried to make the signal to interact with electrons, even from the outside of the evacuated glass bulb. Lee de Forest in 1908 patented his three electrode valve, later known as triode. In the triode a wire bent back and forth to form a sort of gridiron, hence the grid, was placed between the filamentary cathode and the anode. This solution was by far the most accepted one. Anyway a mention goes also the oscillator tube patented in 1912 by Robert Goddard, the rocket pioneer, followed by the production of some unconventional tubes many years later, the C100A and the C100D, made for Collins. Fig. 2 - Drawing of the early Audion triode, from the De Forest patent. Starting from the left, the filamentary cathode, the grid and the plate. Early tubes used filamentary direct-heating cathodes. In many transmitting and high- power tubes tungsten or thoriated-tungsten filaments were retained. Oxide coated filaments were soon preferred in receiving and low-power tube types, due to their higher efficiency which made possible operation at lower temperatures. We must wait until 1925 to see the first indirectly heated cathode, capable of hum-free operation when AC supply sources were used. In the early unipotential cathode tubes introduced by McCullough heater came out from top of the glass bulb. In the twenties we see a parallel growth of AM radio broadcast services and of vacuum tubes, including high-power transmitting types. New materials were investigated and more efficient processes were found to increase performances and yields of vacuum tubes even reducing their costs. Vacuum tube classification When talking of vacuum tubes, usually we just think of the radio receiving types. But vacuum tubes were designed in the past century to fill almost any field of human activity. The huge variety of devices introduced in the years by tube manufacturers makes very difficult to select uniform criteria for their classification, size or type of bulb, external connections, number of electrodes, type of cathode, power, function, principle of operation, application. A first classification could just separate high- vacuum devices from gas-filled types. Another coarse classification could consider the application for which the tube was designed, even if boundaries are not sharply defined: Radio reception, includes tubes designed for radio and/or television sets Radio transmission, with power tubes for AM/FM/TV broadcast transmitters Radar, including many special tubes designed for radar systems. Industrial, including power tubes used in RF heating, welding and anyway in process control Computers, with tubes intended both for analog computing and for logic and interface circuits Measurement, with transducers and signal processing tubes Military and Aerospace, with tubes designed for reliable operation in extreme environments. Display, with CRT tubes and similar devices Electro-medical, including X-ray tubes Power conversion, including tubes designed for power supply circuits The classification given above may be expanded to include other tube families and even glass-sealed passive components. Mainly for receiving tubes another classification could take into account the number of electrodes, as in the table below. Number of electrodes Type name 2 - cathode and anode Diode 3 - cathode, grid and anode Triode 4 - cathode, two grids and anode Tetrode 5 - cathode, three grids and anode Penthode 5 - cathode, two grids, beam forming grid and anode Beam tetrode 6 - Cathode, four grids and anode Exode 7 - Cathode, five grids and anode Pentagrid or Heptode 8 - Cathode, six grids and anode Octode 9 - Multiple units, e.g. diode-triode or twin-triode Referring to the source of electrons we can assume the following classification: Cold cathode, as in some gas-filled tubes Directly heated or filamentary cathode.
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